Terephthalic acid is produced by the liquid-phase oxidation reaction of a p-phenylene compound such as p-alkylbenzenes including p-xylene as a representative example. Usually, in the production of terephthalic acid, a catalyst such as cobalt or manganese is used, or a catalyst obtained by adding an accelerant such as a bromine compound or acetaldehyde to a catalyst such as cobalt or manganese is used, with acetic acid as a solvent.
However, in this liquid-phase oxidation reaction, acetic acid is used as the solvent, the obtained crude terephthalic acid slurry includes, in large amounts, impurities such as 4-carboxybenzaldehyde (hereinafter also described as 4CBA), para-toluic acid (hereinafter also described as p-TOL), and benzoic acid, or other various coloring impurities. The crude terephthalic acid obtained by separating from the crude terephthalic acid slurry also includes these impurities as mixed therein, and thus a considerably higher purification technique is required for the purpose of obtaining high-purity terephthalic acid.
As a method for purifying crude terephthalic acid, there have been known various methods such as dissolving crude terephthalic acid in acetic acid or water, a mixed solvent thereof, or the like at high temperature and high pressure followed by catalytic hydrogenation treatment, decarbonylation treatment, oxidation treatment, or recrystallization treatment, or high temperature immersion treatment in a slurry condition in which terephthalic acid crystals are partially dissolved. When any purification method is used, finally, the operation of separating the terephthalic acid crystals from the mother liquor is required.
Oxidation intermediates such as 4CBA, p-TOL, and benzoic acid or coloring substances which are present as impurities in the oxidation-derived slurry or the slurry resulting from purification of crude terephthalic acid are dissolved in the dispersion medium of the slurry at high temperature. Thus, when the slurry is cooled to around 100° C. to give slurry containing terephthalic acid crystals, these impurities are incorporated in the terephthalic acid crystals, with the result that it is difficult to obtain high-purity terephthalic acid.
Therefore, in order to obtain high-purity terephthalic acid from slurry after the purification treatment of crude terephthalic acid, it is necessary to separate the terephthalic acid from the mother liquor under the conditions of high temperature and high pressure. The method most generally used as a method for separating a mother liquor from slurry comprising terephthalic acid crystals is a centrifugation method. The centrifugation method is a method in which the slurry is introduced into a basket rotating at high speed so as to cause the dispersion medium to overflow from the top of the basket while directing the crystals to the bottom of the basket. It is known that the centrifugation method involves some difficulties in continuous operation at high temperature and high pressure because of the structural and functional restrictions of the centrifuge.
First, the crystals are difficult to rinse during the centrifugation or after the separation, and thus the amount of the dispersion medium adhered to the crystals tends to increase. A common method employed to solve this problem is to form a cake of the centrifugally separated terephthalic acid crystals into slurry with a fresh, hot solvent. This method, however, has the disadvantage of requiring several repetitions of the separation procedure. Further, since high speed rotation is performed at high temperature and high pressure, the maintenance and service of the centrifuge are complicated and difficult, and investment in them increases, and it is difficult to say that the centrifugation method is advanced as a technique in this field.
As a separation technique alternative to the centrifugation, a mother liquor replacement apparatus making use of gravitational sedimentation of terephthalic acid crystals has been proposed. For example, in Patent Literature 1, a mother liquor replacement apparatus within which a lateral plate with a plurality of holes is provided is disclosed, and it is described that when the mother liquor replacement apparatus does not have such a structure, the efficiency of replacement decreases due to the channeling or back mixing of the fluid in the apparatus. In addition, in Patent Literature 2, it is described that by providing in an apparatus a shelf plate forming a slope, the replacement performance improves.
In addition, a mother liquor replacement apparatus requiring no shelf plates is also proposed. Patent Literature 3 describes a dispersion medium replacement apparatus having a simple structure that does not require a shelf plate, characterized in that in a dispersion medium replacement tower in which a terephthalic acid slurry and a dispersion medium for replacement are introduced from the upper portion of the tower and the lower portion of the tower respectively, and the terephthalic acid crystals settling in the tower and the dispersion medium for replacement rising in the tower are brought into contact with each other in a counterflow, a stirring apparatus is provided in the lower portion of the tower, and further the terephthalic acid content in the slurry in the lower portion region is made higher than the terephthalic acid content in the slurry in the middle part region.
Regarding a method for producing purified terephthalic acid without a mother liquor replacement step, Patent Literature 4 describes a process in which crude terephthalic acid is dissolved in water and brought into contact with hydrogen in the presence of a catalyst of a platinum group metal for reduction treatment, this treatment liquid is crystallized to form slurry, and the crystals in the above slurry are separated by a solid-liquid separation apparatus to obtain purified terephthalic acid. It is disclosed that the water content of the dehydrated cake of terephthalic acid obtained by the solid-liquid separation in this process based on the terephthalic acid is 15 to 20% by mass, and when drying is further performed using a fluidized bed dryer, the step of decreasing the water content of the terephthalic acid to 14% by mass or less by a method such as flash drying, pre-drying by a heater, or mixing dry terephthalic acid is necessary. Patent Literature 4 discloses that various solid-liquid separation methods such as a screen bowl type centrifuge, a rotary vacuum filter, and a horizontal belt filter are attempted, but it is difficult to decrease the water content to 14% by mass or less only by solid-liquid separation.
Meanwhile, terephthalic acid is reacted with ethylene glycol or the like and mainly used as a starting material of polyester. When such large particle diameter particles that the particle diameter of terephthalic acid exceeds 200 μm increase too much, the terephthalic acid is likely to remain as an unreacted component, and as a result, the need to increase reaction time arises, and the problem of the increase of by-products arises.
Patent Literature 5 discloses a method for setting the proportion of particle diameters exceeding 210 μm at 10% by mass or less. In the method in Patent Literature 5, the proportion of particle diameters exceeding 210 μm is set at 10% by mass or less by defining crystallization temperature in a first crystallization vessel and the range of the stirring power of a stirring blade when dissolving crude terephthalic acid in an aqueous medium, subjecting the solution to catalytic hydrogenation treatment with a platinum group metal catalyst, and performing cooling in stages in multiple stages of crystallization vessels connected in series for crystallization.